Non-destructive stringer inspection apparatus and method

ABSTRACT

A hat stringer inspection device permits continuous inspection of hat stringers as one or more probes are moved along the length of the hat stringer. Probes may be magnetically coupled to opposing surfaces of the structure, including, for example, where one of the probes is positioned inside the hat stringer and the probes are magnetically coupled across the surface of the hat stringer. The device may be autonomous with a feedback-controlled motor to drive the inspection device along the hat stringer. Magnetic coupling is also used to re-orient the position and/or alignment of the probes with respect to changes in the hat stringer or shapes, sizes, and configurations of hat stingers.

CROSS-REFERENCE TO RELATED APPLICATIONS

The contents of U.S. Pat. No. 6,722,202 and co-pending application Ser.No. 10/752,890, entitled “Non-Destructive Inspection Device forInspection Limited-Access Features of a Structure,” filed Jan. 7, 2004;application Ser. No. 10/943,135 entitled “Magnetically AttractedInspecting Apparatus and Method Using a Fluid Bearing,” filed Sep. 16,2004; application Ser. No. 10/943,088, entitled “Magnetically AttractedInspecting Apparatus and Method Using a Ball Bearing,” filed Sep. 16,2004; and application Ser. No. 10/943,170, entitled “AlignmentCompensator for Magnetically Attracted Inspecting Apparatus and Method,”filed Sep. 16, 2004, are incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to an apparatus and method forinspecting a structure and, more particularly, to an apparatus andmethod for non-destructive inspection of limited-access features of astructure.

BACKGROUND

Non-destructive inspection (NDI) of structures involves thoroughlyexamining a structure without harming the structure or requiring itssignificant disassembly. Non-destructive inspection is typicallypreferred to avoid the schedule, labor, and costs associated withremoval of a part for inspection, as well as avoidance of the potentialfor damaging the structure. Non-destructive inspection is advantageousfor many applications in which a thorough inspection of the exteriorand/or interior of a structure is required. For example, non-destructiveinspection is commonly used in the aircraft industry to inspect aircraftstructures for any type of internal or external damage to or flaws inthe structure. Inspection may be performed during manufacturing of astructure and/or once a structure is in-service. For example, inspectionmay be required to validate the integrity and fitness of a structure forcontinued use in manufacturing and future ongoing use in-service.However, access to interior surfaces is often more difficult orimpossible without disassembly, such as removing a part for inspectionfrom an aircraft.

Among the structures that are routinely non-destructively tested arecomposite structures, such as composite sandwich structures and otheradhesive bonded panels and assemblies, such as hat stringers or hatstiffeners made from carbon fiber reinforced and graphite epoxy (Gr/Ep)materials and co-cured or co-bonded hat stringers. In this regard,composite structures are commonly used throughout the aircraft industrybecause of the engineering qualities, design flexibility and low weight,such as the stiffness-to-weight ratio. As such, it is frequentlydesirable to inspect composite structures to identify any flaws, such ascracks, voids or porosity, which could adversely affect the performanceof the composite structure.

Various types of sensors may be used to perform non-destructiveinspection. One or more sensors may move over the portion of thestructure to be examined, and receive data regarding the structure. Forexample, a pulse-echo (PE), through transmission (TT), or shear wavesensor may be used to obtain ultrasonic data, such as for thicknessgauging, detection of laminar defects and porosity, and/or crackdetection in the structure. Resonance, pulse echo or mechanicalimpedance sensors may be used to provide indications of voids orporosity, such as in adhesive bondlines of the structure. Highresolution inspection of aircraft structure are commonly performed usingsemi-automated ultrasonic testing (UT) to provide a plan view image ofthe part or structure under inspection. For example, solid laminates maybe inspected using one-sided pulse echo ultrasonic (PEU) testing andcomposite sandwich structures may be inspected using two-sidedthrough-transmission ultrasonic (TTU) testing. In pulse echo ultrasonic(PEU) testing, ultrasonic sensors, such as ultrasonic transducers, arepositioned adjacent to or near one surface of the structure to beinspected. For example, the PEU transducer transmits an ultrasonicsignal into the structure under inspection and receives the reflectionof the ultrasonic signal from the structure. In through-transmissionultrasonic inspection, paired ultrasonic sensors such as transducers, ortransducer and a receiver pairings, are positioned facing the other butcontacting opposite sides of the structure. An ultrasonic signal istransmitted by at least one of the transducers, propagated through thestructure, and received by the other transducer. Data acquired bysensors, such as PEU and TTU transducers, is typically processed by aprocessing element, and the processed data may be presented to a uservia a display. A data acquisition board and data handling software maybe used for collection and display of inspection data, such asdisplaying the data on a computer monitor as an image representation ofthe structure under inspection, such as a hat stringer, supplementedwith corresponding color and/or graphical data of the inspection topermit examination by a qualified inspector.

Non-destructive inspection may be performed manually by technicians whotypically move an appropriate sensor over the structure. Manual scanningrequires a trained technician to move the sensor over all portions ofthe structure needing inspection. Manual scanning typically involves thetechnician repeatedly moving a sensor side-to-side in one directionwhile simultaneously indexing the sensor in another direction. Inaddition, because sensors typically do not associate locationinformation with the acquired data, the same technician who is manuallyscanning the structure must also watch the sensor display while scanningthe structure to determine where the defects, if any, are located in thestructure. The quality of the inspection, therefore, depends in largepart upon the technician's performance, not only regarding the motion ofthe sensor, but also the attentiveness of the technician in interpretingthe displayed data. Thus, manual scanning of structures istime-consuming, labor-intensive, and prone to human error.

Semi-automated inspection systems have also been developed. For example,the Mobile Automated Scanner (MAUS®) system is a mobile scanning systemthat generally employs a fixed frame and one or more automated scanningheads typically adapted for ultrasonic inspection. A MAUS system may beused with pulse-echo, shear wave, and through-transmission sensors. Thefixed frame may be attached to a surface of a structure to be inspectedby vacuum suction cups, magnets, or like affixation methods. SmallerMAUS systems may be portable units manually moved over the surface of astructure by a technician.

Automated inspection systems have also been developed. For example, theAutomated Ultrasonic Scanning System (AUSS®) system is a complexmechanical scanning system that may employ through-transmissionultrasonic inspection. An AUSS system can also perform pulse echoinspections, and simultaneous dual frequency inspections. The AUSSsystem has robotically controlled probe arms that may be positioned, forexample, for TTU inspection proximate the opposed surfaces of thestructure undergoing inspection with one probe arm moving an ultrasonictransmitter along one surface of the structure, and the other probe armcorrespondingly moving an ultrasonic receiver along the opposed surfaceof the structure. To maintain the ultrasonic transmitter and receiver inproper alignment and spacing with one another and with the structureundergoing inspection, a conventional automated inspection system mayhave a complex positioning system that provides motion control innumerous axes, such as the AUSS-X system which has motion control in tenaxes. Automated inspection systems, and like robotics, however, can beprohibitively expensive. Further, orienting and spacing sensors withrespect to the structure, and with respect to one another for TTUinspection, may be especially difficult in conjunction with structureswith non-planar shapes, such as the inspection of curved structures andhat stringers. Also, conventional automated scanning systems, such asthe AUSS-X system, may require access to both sides of a structurewhich, at least in some circumstances, will be problematic, if notimpossible, particularly for very large or small structures.Furthermore, scanning systems inspect limited areas up to a few meterssquare.

Accessibility to the structure requiring inspection and to particularfeatures is also an important consideration. Access may be so limitedthat manual inspection or automated inspection is not possible. Forexample, the inside of a hat stringer of the fuselage of an aircraft haslimited access for inspection, especially far from an end.

SUMMARY OF THE INVENTION

An apparatus for inspecting a structure, such as a hat stringer,includes autonomous crawlers coupled to the fuselage to inspect the hatstringers using a feedback-controlled motor to drive the inspectiondevice. The present system is particularly adapted for inspectingcomposite hat stringers.

Magnetic coupling holds the inspection device on the structure underinspection on opposite sides for through transmission inspection. Thefirst probe of the inspection device rides along the outside of the hatstringer while the second probe rides below the hat stringer or insidethe hat stringer. The magnetic coupling along with the configuration ofthe inspection device keep inspection sensors, such as ultrasonictransducers or x-ray sources and detectors, are aligned to inspect thehat stringer as the inspection device moves along the length of the hatstringer. The magnetic coupling keeps the transducers coupled and inphysical contact with the surfaces of the structure. Keeping thetransducers coupled to the structure is important to ensure reliableinspection, such as to provide for strong and consistent signaltransmission. The design of the inspection device also permits accurateposition measurement or sensing along the length of the structure byusing an encoder.

Embodiments of the present invention refine general features andfunctionality of the Remote Access Bondline Inspection Tool (RABIT) asdescribed in U.S. patent application Ser. No. 10/752,890, filed Jan. 7,2004, for inspecting hat stringers.

The preferred design accommodates variations of different hat stringerdesign shapes and sizes and part thickness changes, such as axial partthickness variance. For example, a central section and side sections ofan interior probe may be arranged to permit the side sections to move tomatch the angle of the sides of the hat stringer. The probe might beself-adjusting using a spring loading or more sophisticated adjustmentmeans like adjustment slots or jackscrews. Transducers oriented toinspect the corners of the hat stringer will remain aligned with thecorners by telescoping a middle section as the hat stringer increases inwidth. Selection of transducers depends on the type of material and itsthickness. Higher frequency transducers usually are used for thinner hatstringer thicknesses.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a perspective view of a structure with two hat stringers.

FIG. 2 is an isometric view of a hat stringer.

FIG. 3 is a cross-sectional schematic diagram of an inspection apparatusof the present invention for inspecting a hat stringer.

FIG. 4 is a cross-sectional schematic diagram of another embodiment ofthe inspection apparatus.

FIG. 5A is a bottom plan view of a portion of an inspection probe.

FIG. 5B is a cross-sectional schematic diagram of a portion of the probeof FIG. 5A.

FIG. 6A is a cross-sectional schematic diagram of a portion of anotherinspection probe.

FIG. 6B is a cross-sectional schematic diagram of a portion of the probeof FIG. 6A.

FIG. 6C is another cross-sectional schematic diagram of a portion of theprobe of FIG. 6A.

FIG. 7 is a perspective view of another inspection probe.

FIG. 8 is a perspective view of yet another inspection probe.

FIGS. 9, 10, 11, and 12 are perspective views of the inspection probe ofFIG. 8.

FIGS. 13, 14, and 15 are perspective views of yet other inspectionprobes.

FIGS. 16 and 17, respectively, are side elevation and cross-sectionalviews of the probe of FIG. 15.

FIG. 18 is a perspective view of yet another inspection probe.

FIGS. 19, 20, 21, and 22 are perspective views of the probe of FIG. 18.

FIG. 23A is a right side elevation view of yet another probe.

FIG. 23B is a bottom plan view of the probe of FIG. 23A.

FIG. 23C is a left side elevation view of the probe of FIG. 23A.

FIG. 23D is a front elevation view of the probe of FIG. 23A.

FIG. 23E is a top plan view of the probe of FIG. 23A.

FIG. 23F is a partial front elevation view of the probe of FIG. 23A.

FIG. 23G is a partial bottom plan view of the probe of FIG. 23A.

FIG. 23H is a partial right side elevation view of the probe of FIG.23A.

FIG. 24 is a perspective view of yet another inspection probe.

FIGS. 25 and 26 are perspective views of the probe of FIG. 24.

FIG. 27 is a perspective view of yet another inspection probe.

FIGS. 28, 29, and 30 are perspective views of the probe of FIG. 27.

FIG. 31A is a cross-sectional schematic diagram of the probe of FIG. 27.

FIG. 31B is a cross-sectional plan view of the probe of FIG. 27.

FIG. 32A is a cross-sectional schematic diagram of a section of theprobe of FIG. 27 as indicated by A-A in FIG. 31B.

FIG. 32B is a cross-sectional schematic diagram of a section of theprobe of FIG. 27 as indicated by B-B in FIG. 31B.

FIG. 32C is a cross-sectional schematic diagram of a section of theprobe of FIG. 27 as indicated by C-C in FIG. 31B.

FIG. 32D is a cross-sectional schematic diagram of a section of theprobe of FIG. 27 as indicated D-D in FIG. 31B.

FIG. 32E is a cross-sectional schematic diagram of a section of theprobe of FIG. 27 as indicated E-E in FIG. 31B.

DETAILED DESCRIPTION

The present invention will be described more fully with reference to theaccompanying drawings. Some, but not all, embodiments of the inventionare shown. The invention may be embodied in many different forms andshould not be construed as limited to the described embodiments. Likenumbers and variables refer to like elements and parameters throughoutthe drawings.

Embodiments of non-destructive stringer inspection apparatus and methodsof the present invention are described with respect to hat stringers,especially composite hat stringers for an aircraft fuselage. However,the apparatus and methods may also be used for similar applicationswhich require non-destructive inspection, including other compositestructures with difficult-to-inspect geometric configurations and/orremote locations. Embodiments of hat stringer inspection apparatus andmethods may include magnetically coupled probes as described inco-pending application Ser. Nos. 10/943,088; 10/943,135; 10/752,890; or10/943,170.

Inspection devices can inspect a variety of structures formed of variousmaterials. For devices which transmit magnetic fields through thestructure, however, the structure under inspection is preferablynon-magnetic, that is, the structure preferably has no magneticpermeability. Such structures include composites such as carbon fiber orgraphite reinforced epoxy (Gr/Ep) and non-ferromagnetic metals (e.g.aluminum alloy, titanium alloy, or aluminum or titanium hybrid laminatessuch as GLARE or Ti/Gr). The surfaces and intermediate septums whichcollectively define the test article are non-magnetic to allow magneticcoupling between the probes.

Inspecting hat stringers normally requires a one-sided inspectiontechnique, such as pulse echo ultrasonic (PEU) inspection. However, theshapes of hat stringers complicate the inspection. The hat stringerinspection device can perform pulse echo inspection or throughtransmission ultrasonic (TTU) inspection.

Inspection sensors of a probe may be strategically placed and oriented,such as aiming transducers at the corners or edges of the hat stiffener,to ensure full inspection of the entire hat stringer. Support structuresfor inspection sensors, also referred to as transducer holders, may befabricated for specific placement and orientation of inspection sensorscorresponding to the intended shapes and sizes of hat stringers. Forexample, the first inspection probe 80 with transducer holders 84 shownin FIGS. 8-12 may be used for handheld pulse echo (PE) inspection usingtwelve inspection sensors 38 on one side of the hat stringer and sixinspection sensors 38 on the top section of the hat stringer. Inspectionsensors located near the intersecting corner of the hat stringer betweenthe top section and one of the sides may be oriented to inspect thecorner. Alternatively, or in addition, inspection sensors may beincluded between the side and top to inspect a corner of the hatstringer. An inspection device using the transducer holders 84 shown inFIGS. 8-12 would require scanning the hat stringer once along each sideof the hat stringer. Other embodiments may be configured to scan bothsides and the top section of a hat stringer to permit single-passinspection.

Three types of inspection probes are provided. A fixed orientation probeincludes a housing that supports any of inspection sensors, contactmembers, and magnetic coupling devices such that the inspection probefunctions as a single, integrated device. A segmented probe decouplesthe housing that supports any contact members and/or magnetic couplingdevices from transducer holders that support the inspection sensors suchthat the probe is capable of re-orienting the transducer holders and,thereby, any inspection sensors with the shape and structure of the hatstringer under inspection. A partially fixed orientation probe mayinclude hinged sections which permit limited re-orientation of theinspection probe to permit better alignment of inspection sensors orpositioning of magnetic coupling devices and/or contact members. Thesethree variations of inspections probes may be used in differentcombinations depending on the particular circumstances of inspection.

FIG. 1 is a perspective view of a structure with two hat stringers.Their structure includes a skin 12 to which individual or connected hatstringers 10 may be attached to stiffen for the overall structure. InFIG. 2, the hat stringer has an outside surface 16 and an inside surface18. The skin 12 has an upper surface 15 and a lower surface 14. The hatstringer 10 is a trapezoidal structure and is affixed to the skin 12 atcorners or edges 20, 26. The hat stringer 10 has corners or edges 22, 24that are lifted off of the skin.

If access is available under the hat stringer at the exterior surface ofthe fuselage, magnetic coupling through the fuselage wall will hold thefirst probe on the hat stringer and the second probe against theinterior surface of the fuselage, as shown in FIG. 3. If access is notavailable to the exterior surface of the fuselage and/or the hatstringer has an open end to allow insertion and removal of an interiorprobe of the inspection device, magnetic coupling occurs through the hatstringer, as shown in FIG. 4. With access to the interior, throughtransmission ultrasonic (TTU) inspection techniques may be used.

In FIG. 3, probe 40 rides along a surface 14 of the skin 12. Probe 30rides along the outer surface 16 of the hat stringer 10 and a surface 15of the skin 12. A probe which rides inside a hat stringer may bemagnetically coupled to a probe which rides on the outside of the hatstringer. Each probe 30, 40 may include one or more magnetic couplingdevices 42 such as magnets or ferromagnetic material inserts to providea magnetic coupling between the two probes 30, 40 to hold the probes 30,40 in alignment and to provide leader-follower motion in tandem.Magnetic coupling may be adjusted by changing the size and/or strengthof a magnet.

At least one probe may include inspection sensors such as a pulse echosensor, ultrasonic transducer, x-ray source, x-ray detector, encoder, orcamera. The ultrasonic transducer may be a 1 MHz immersion transducerfrom Agfa/Krautkramer of Lewistown, Pa. As shown in FIG. 3, one probe 30includes ultrasonic transducers 36, 37, 38, 39 oriented to inspectvarious components of the hat stringer 10. Transducer 36 inspects acorner 24. Transducer 37 inspects a corner 26 that attaches to the skin12. Transducers 38 inspect the side surface. Transducers 39 inspect theportion of the hat stringer 10 furthest from the skin 12.

The probes 30, 40 also include contact members 41 such as wheels,bearings, or skids to allow the probes 30, 40 to easily move acrosssurfaces 14, 15 and 16. Each contact member 41 supports the probe 30, 40at the proper spacing from the surface 14, 15 and 16 and reduces thefrictional drag to permit smooth translation of the probe across thesurface.

By magnetically coupling the probes 30, 40 to each other on opposingsurfaces 14, 15 and 16 of the structure under inspection, either probemay be driven, i.e., moved across the respective surface of thestructure under inspection by a translational force, thereby moving theopposing magnetically coupled probe. Each probe 30, 40 includes ahousing 34, 44 to carry the various elements of the probe, such as themagnetic coupling devices 42, contact members 41, and inspection sensors36, 37, 38, 39. The housing may be made, for example, from plexiglass,plastic, or other materials which provide structural support,durability, do not interfere with the inspection method, and areunlikely to damage the surface of the part under inspection such as byscratching the surface upon contact.

A probe 30 may be designed to accommodate hat stringers of differentwidths and hat stringers that vary in width. A probe 30 may be formedfrom two sides 46, 48. As the width of a hat stringer increases or whenthe inspection probe 30 is placed on a hat stringer with a large width,the probe 30 can separate into the first and second sides 46, 48. Thefirst and second sides 46, 48 may be magnetically coupled to each othersuch as using magnetic coupling devices to support the first and secondsides 46, 48 against the outside surface 16 of the hat stringer andprevent the first and second sides 46, 48 from separating more than thewidth of the hat stringer under inspection.

In FIG. 4, a probe 40 is located inside the hat stringer 10 and issupported against the inside surface 18 of the hat stringer 10.

FIG. 5A is a bottom plan view of a portion of an inspection probe; FIG.5B is a cross-sectional schematic diagram of the portion of the probe ofFIG. 5A. The probe shown in FIGS. 5A and 5B includes magnetic couplingdevices 42 to support the probe against a hat stringer and inspect aside and a top portion of the hat stringer. The probe may be attached toor integrally formed with the portion of a probe shown in FIGS. 6A, 6B,and 6C. The probe shown in FIGS. 6A, 6B, and 6C provides for movementalong the length of a hat stringer for continuous inspection. Theportion of an inspection probe shown in FIGS. 6A, 6B, and 6C may bereferred to as a structural member, frame, body, or exoskeleton forsupporting an a probe. A housing 60 may support a motor 62 which drivesan axle 70. The axle may turn one or more contact members 64 whichsupport the housing 60 against a structure. The axle 70 may also supportgears 68 which drive other contact members 66.

FIG. 7 is a perspective view of another inspection probe 70 that ridesbelow or inside a hat stringer. The probe 70 includes contact members72, 74, and magnetic coupling devices 42.

In FIGS. 8, 9, 10, 11, and 12, a probe 80 can inspect two surfaces of ahat stringer when moved in one direction and the remaining side whenscanned in the opposite direction. Transducer holders 84 support andalign the inspection sensors 38. Recesses 136, 138, 139 in thetransducer holders 84 receive the inspection sensors and position thesensors to achieve complete scanning of hat stringer surfaces. Recess139 is located to align a sensor to scan the top surface of a hatstringer. Recess 136 aligns a sensor to scan the corner or edge of thehat stringer. An end recess 138 positions a sensor to scan the sidesurface of the hat stringer.

For ultrasonic inspection, a fluid, like water or air, can be fedthrough supply lines into channels on the inspection device to dispersethe water between the device and the structure and to couple testsignals between the inspection device and the structure. A fluid inlet86 permits transfer of a fluid through the housing 82 to channels todisperse the fluid. The design of FIG. 8 maintains a fluid coupling pathbetween inspection sensors 38 and the hat stringer when only a portionof the inspection probe 80 is over the hat stringer as shown in FIGS. 11and 12. Advantageously, the fluid flows smoothly without bubbles,cavitation, or turbulence that would detrimentally affect the signal tonoise ratio.

Also, instead of contact members, a fluid bearing may be created bypumping a thin layer of fluid, like water or air, between the housingand the surface of the structure. A fluid bearing may further preventscratching of the surface.

Even if the probe has a fluid bearing, it may also include one or morecontact members. Skids may be beneficial for fluid bearing probes toprevent damage or marring of a surface of a structure when initiallyplacing a probe on the structure or magnetically coupling two probes onopposite sides of the structure, particularly when the fluid bearing isnot be in use.

In FIG. 13, a probe 200 includes three separate transducer holders 202,204, 206 which each include magnetic coupling devices 42 and recesses138, 139 for inspection sensors. By separating each transducer holder202, 204, 206, the probe 200 is capable of re-orienting each transducerholder 202, 204, 206 and any inspection sensors supported thereby as maybe required for inspecting hat stringers with different shapes,different angles along the sides of the hat stringer, and/or differentheights of the hat stringer. The probe 200 is used with a probe 90 asshown in FIG. 14. Probe 90 includes hinges 92, or other rotatablemechanics, that permit the side portions 93, 94 of the second inspectionprobe 90 to rotate to match the corresponding angle of the sides of thehat stringer. As such, the magnetic attraction between the magneticcoupling devices 42 of the first and second probes 200, 90 permitre-orientation of the side transducer holders 202, 204 to match thecorresponding angle of the sides of the hat stringer. Accordingly, theside transducer holders 202, 204 of the first inspection on probe 200match the angle of the outer surface 16 of the hat stringer 10 and theside portions 93, 94 of the second inspection probe 90 match the angleof the inner surface 18 of the hat stringer 10.

FIGS. 15, 16, and 17 show views of yet another inspection probe of thepresent invention. The embodiment of the inspection probe shown in thesefigures is akin to the portion of an inspection probe shown in FIGS. 6A,6B, and 6C such that the housing 260 of the first inspection probe 210is decoupled from the transducer holders or inspection shoes 202, 204,206 of the first inspection probe 210. The housing, or exoskeleton, 260of the first inspection probe 210 supports various components for thefirst inspection probe 210 to provide for support, alignment, andtranslation of the first inspection probe 210 along the length of a hatstringer for continuous inspection. A motor 262 is used to drive contactmembers 266 such as friction wheels. Gears 268 may be used to transferthe rotation of the motor 262 to the contact members 266 using a drivechain 269 or like translational means and an axle 270 for geartransmission. The first inspection probe may also include a linearencoder 261 such as an optical shaft encoder, a linear encoder, anoptical sensor, a directional sensor, or wheel encoder that may bemounted to the housing 260 of the first inspection probe 210 to providefeedback of the position, speed, direction, and/or velocity of the firstinspection probe 210. Embodiments of the present invention may use asmart stepper motor that drives the wheels and an optical encoder todenote location and speed of the inspection device. For example, aninspection device may be capable of inspecting at a rate as much as ormore than ten feet in length of a hat stringer per minute, and a smartstepper motor can operate with an electronic controller, such as an acomputer with control software, to move the inspection device in amanner advantageous to the speed of the scanning technology in use.

The first inspection probe 210 may cooperate with a second inspectionprobe 211 as seen in FIG. 17. Transducer holders, or shoes, 202, 204,206 of the first inspection probe 210 may include magnetic couplingdevices 42 which correspond with magnetic coupling devices 42 of thesecond inspection probe 211. The magnetic attraction between thecorresponding magnetic coupling devices 42 of the first and secondinspection probes 210, 211 provides for support and orientation of thetransducer holders 202, 204, 206 along the outside surface of the hatstringer. The second inspection probe 211 and the transducer holders202, 204, 206 may include contact members 41. To maintain properalignment of the transducer holders 202, 204, 206, decoupling supportsare used to adjust the alignment or orientation of the transducerholders 202, 204, 206. Probe 210 includes springs 213 and a verticaladjustment drive 212.

Probe 210 straddles the hat stringer to permit movement of the firstprobe 210 along the length of the hat stringer for inspection. Probe 211is inside the hat stringer in the application of FIG. 17. Magneticcoupling devices 42 in the first and second probes 210, 211 providemagnetic attraction between the probes 210, 211 to maintain contact withopposing surfaces of the structure and corresponding relationalpositions with respect to the other probe, including during cooperativemovement of the probes 210, 211. Contact members 41 of the probes 210,211 permit controlled movement of the probes 210, 211 over therespective surfaces of the structure for inspection of the hat stringer.Inspection sensors, such as ultrasonic transducers, are activated toinspect the hat stringer as the probes 210, 211 are moved along thelength of the hat stringer by the motor 262. Alternatively, movement ofthe probes may be provided by an automated control system or by manualoperation. A couplant may be used to improve the quality of theinspection of the hat stringer.

As the inspection device moves along the length of the hat stringer, theprobes 210, 211 may re-orient the magnetic coupling devices 42 and/orinspection sensors to the shape of the hat stringer such as tocompensate for a change in the angle of the sides of the hat stringer.

In FIGS. 18, 19, 20, 21 and 22, probe 300 includes adjustable magneticcoupling devices 342 at either end. The magnetic coupling device 342 canadjust using a slidable portion 302 which can account for differentheights of hat stringers. As more clearly visible in FIGS. 21 and 22,the inspection probe 300 includes two transducer holders 304, 306. Oneof the transducer holders 304 is configured to provide for positioningand alignment for scanning a side surface of a hat stringer. The othertransducer holder 306 is positioned to provide alignment for scanningthe top surface of a hat stringer. Corresponding recesses 338, 339permit an array of inspection sensors to be located on each of thesetransducer holders 304, 306. The inspection probe 300 is configured todecouple the transducer holders 304, 306 from the housing 360 to permitthe transducer holders 304, 306 and inspection sensors to reorient withrespect to the surface being scanned. For example, the transducer holder304 configured to scan the side of a hat stringer is attached to asupport element 314 which is attached to the housing 360 withspring-biased dampers 318 which permit the support element 314 toachieve, to a certain extent, three degrees of freedom with respect tothe housing 360. Spring-biased dampers 318 may be linear slides havinglinear rods with bearings to provide for movement toward and away fromthe surface of the hat stringer in addition to angular freedom of motionfor aligning to the surface of the hat stringer. The transducer holder304 is attached to the support element 314 by a hinged attachment 315which permits an additional degree of freedom. The transducer holders304, 306 have magnetic coupling devices that couple the transducerholders 304, 306 to the surface of the hat stringer by attraction tomagnetic coupling devices of the probe inside the hat stringer. In FIGS.23A-H, airpots or dashpots 312, 313 are included to dampen vibrationsand/or oscillations which may deteriorate the inspection quality fromthe inspection sensors located on the decoupled transducer holders 304,306.

In FIGS. 24, 25 and 26, a probe 400 is separated into first and secondsides 402, 404 to permit the inspection probe 400 to achieve variouswidths. Magnetic coupling devices 442 provide magnetic attraction ofprobe 400 to support probe 400 against the surface of the hat stringer.

In FIGS. 27, 28, 29, 30, 31A, 31B, and 32A-E, a probe 500 includes amotor 462 surrounded by a motor housing 463 which drives the probe 500along the surface of the structure under inspection to providecontinuous scanning. For example, the motor may drive contact memberssuch as wheels 466 which drive a tread 440 that rides along the surfaceof the skin. The motor 468 may translate its rotational force from agear 468 to the contact members using a drive chain 469 such as a beltor chain. Configurations of inspection sensors may include arrayspositioned on the inspection probe 500 to scan the entire surface of ahat stringer.

The invention should not be limited to the specific disclosedembodiments. Specific terms are used in a generic and descriptive senseonly and not for purposes of limitation.

1. A non-destructive inspection apparatus for inspecting a structure,comprising: a first probe configured for traveling over a first surfaceof the structure under inspection, the first probe comprising at leastone inspection sensor for inspecting the structure as the first probe ismoved over the first surface of the structure and at least one magneticcoupling device, wherein the first probe is further configured to span across-section of the structure that protrudes from the first surface ofthe structure; and a second probe configured for traveling over a secondsurface of the structure and magnetically coupled to the first probe,the second probe comprising and at least one magnetic coupling device,wherein the probes cooperate by the magnetic attraction betweencorresponding magnetic coupling devices to correspondingly move themagnetically coupled probes over the respective surfaces of thestructure.
 2. The apparatus of claim 1, wherein the first probe isfurther configured to span three sides of a cross-section of thestructure that protrudes from the first surface of the structure to forma shape selected from the group of: a trapezoid, a box, a triangle, apartial ellipse, a partial circle, a partial oval, and a catenary curve.3. The apparatus of claim 1, wherein the first probe is furtherconfigured to span a cross-section of the structure that protrudes fromthe first surface of the structure to form a hat stringer.
 4. Theapparatus of claim 1, wherein the second probe is further configured tofit within a cross-section of the structure that protrudes from thefirst surface of the structure to form a hat stringer.
 5. The apparatusof claim 1, wherein one of the probes further comprises a motor formoving the probe over the respective surface of the structure andthereby moving the magnetically coupled probe over the opposing surfaceof the structure by the magnetic attraction of the magnetic couplingbetween the first and second probes.
 6. The apparatus of claim 1,wherein the first probe is separated into a first side and a secondside, and wherein each of the first side of the first probe and thesecond side of the first probe comprise at least one inspection sensor.7. The apparatus of claim 6, wherein each of the first and second sidesof the first probe comprises at least one magnetic coupling devicepositioned to be magnetically coupled with a corresponding magneticcoupling device of the second side and the first side, respectively,thereby holding the first and second sides against the first surface ofthe structure.
 8. The apparatus of claim 6, wherein each of the firstprobe and the second probe comprise at least one magnetic couplingdevice positioned to be magnetically coupled with a correspondingmagnetic coupling device of the second probe on either side of thecross-section of the structure which the first probe spans.
 9. Theapparatus of claim 1, wherein at least one of the first probe and thesecond probe comprises at least one contact member for contacting therespective surface of the structure and upon which the respective proberides for translating over the respective surface of the structure,wherein the contact members are selected from the group consisting awheel, a ball bearing, a fluid bearing, a skid, and a tread.
 10. A probefor inspecting a structure comprising: a housing configured fortraveling over a first surface of the structure under inspection, andwherein the housing is further configured to span a cross-section of thestructure that protrudes from the first surface of the structure; and atleast one inspection sensor carried by the housing for inspecting thestructure as the probe is moved over the first surface of the structure.11. The probe of claim 10, wherein the housing is further configured tospan three sides of a cross-section of the structure that protrudes fromthe first surface of the structure to form a trapezoid.
 12. The probe ofclaim 10, wherein the housing is further configured to span across-section of the structure that protrudes from the first surface ofthe structure to form a hat stringer.
 13. The probe of claim 12, whereinthe probe further comprises at least one inspection sensor oriented toinspect a first edge of the hat stringer and at least one inspectionsensor oriented to inspect a second edge of the hat stringer adjacent tothe first corner.
 14. The probe of claim 13, wherein the probe furthercomprises at least one inspection sensor oriented to inspect a thirdedge of the hat stringer adjacent to the second edge and at least oneinspection sensor oriented to inspect a fourth edge of the hat stringeradjacent to the third edge.
 15. The probe of claim 10, wherein the atleast one inspection sensor of the probe comprises an array ofinspection sensors.
 16. The probe of claim 15, wherein the probe furthercomprise at least one transducer holder connected to the housing and forsupporting and orienting the array of inspection sensors.
 17. The probeof claim 10, wherein the probe is separated into a first side and asecond side, and wherein each of the first side of the probe and thesecond side of the probe comprise at least one inspection sensor. 18.The probe of claim 17, wherein each of the first and second sides of theprobe comprises at least one magnetic coupling device positioned to bemagnetically coupled with a corresponding magnetic coupling device ofthe second side and the first side, respectively, thereby holding thefirst and second sides against the first surface of the structure. 19.The probe of claim 10, wherein the probe further comprises a motorconnected to the housing and for moving the probe over the first surfaceof the structure.
 20. The probe of claim 10, wherein the probe furthercomprises at least one magnetic coupling device carried by the housing.21. A method of inspecting a structure comprising: supporting a firstprobe on a first surface of the structure and a second probe on anopposed second surface of the structure, wherein the first probe isconfigured to span a cross-section of the structure that protrudes fromthe first surface of the structure to form a hat stringer; positioningthe first probe across the hat stringer; establishing magneticattraction between the first and second probes such that the first andsecond probes are drawn toward the first and second surfaces of thestructure, respectively; moving one probe along one surface of thestructure which causes the other probe to move along the opposingsurface of the structure; and transmitting inspection signals into andreceiving inspection signals from the structure as the probes are movedalong the structure.
 22. The method of claim 21, wherein the secondprobe is configured to fit within a protruding cross-section of thestructure forming a hat stringer, and wherein the method furthercomprises the step of positioning the second probe within the hatstringer.
 23. The method of claim 22, wherein the second probe isfurther configured to fit within the hat stringer when the hat stringeris attached to a skin of the structure.
 24. The method of claim 21,further comprising the step of re-orienting one or more inspectionsensors or magnetic coupling devices of at least one of the first andsecond probes with the respective surface of the hat stringer.
 25. Themethod of claim 21, further comprising the steps of separating the firstprobe into first and second sides to straddle the hat stringer, whereinthe hat stringer has a width greater than the configuration of the firstprobe, and establishing magnetic attraction between the first and secondsides of the first probe such that the first and second sides are drawntowards each other and, thereby, maintaining the support of the firstprobe on the first surface of the structure.